Various types of apparatuses and methods have been proposed heretofore for forming plastic pipe structures, such as pipe joints or fittings, of synthetic resin having an internal peripheral groove therein. It is generally economically advantageous to form the internal groove in the pipe structure before removing the same from the mold and while the synthetic resin material is in a plastic state, as opposed to forming the internal groove by machinery after removing the pipe structure from the mold.
An apparatus for injection molding a plastic pipe, and forming an internal peripheral groove for accommodating a seal ring therein, without removing the plastic pipe from the molding apparatus until after the internal groove has been formed, is disclosed in U.S. Pat. No. 4,100,307 (Asano), for example. According to the disclosure in this patent, an internal sleeve is axially movable within the mold and relative to an internal core, following injection of synthetic resin material into the mold and around the core, so as to form an annular cavity or opening around the plastic body, whereupon pressurized fluid, preferably in the form of compressed air, is introduced through a peripheral passage of the core toward the internal periphery of the plastic body to displace the body into the annular cavity and thereby form an internal annular groove in the plastic body. However, the passage through which the pressurized fluid is introduced is formed following the molding of the plastic pipe by moving an auxiliary core member, which is axially aligned with a main body of the core and is normally in end-to-end abutting relation to the main core body, axially away from the main core body.